Medium balls fit medium sized Ball Launchers. 
Small balls fit in the Chuckit Small Laucher. 
Large ball will fit in Large Chuckit Launcher. 

2 balls per pack of Small and Medium. 
Large ball only 1 ball per pack. 
Best balls you will ever buy. 

High bounce
Long range
Ultra durable natural rubber
Floats in water 
Easy to clean
Same size as a tennis ball

Chuckit Ultra Ball - small, medium and large